Gas bearing system

ABSTRACT

A gas bearing system comprising two opposing and substantially parallel bearing surfaces ( 2,4 ) and at least one gas duct ( 6 ) for supplying gas to the bearing gap ( 5 ) between said bearing surfaces ( 2,4 ). A cavity ( 10,14,15,18,19 ) having a content between 0.001 cm3 and 0.2 cm 3  is present, which cavity is connected to said bearing gap ( 5 ) through an orifice ( 7,11,16,17,20,21 ). The bearing surfaces ( 2,4 ) may comprise a recessed area ( 13 ) in which the distance between said two bearing surfaces ( 2,4 ) is larger than the distance between said two bearing surfaces ( 2,4 ) in the portion of said bearing gap ( 5 ) surrounding said recessed area ( 13 ).

The invention is related to a gas bearing system comprising two opposingand substantially parallel bearing surfaces and at least one gas ductfor supplying gas to the bearing gap between said bearing surfaces.

The gas bearing system may have substantially flat bearing surfaces, sothat it can be used to support and guide a member making a translatingmovement. Such gas bearing systems are frequently used as guiding andsupporting elements in high precision machines. The bearing surfaces mayalso have a cylindrical shape, enabling a rotating member to besupported. Also other shapes—adapted to the relative movement of thebearing surfaces—are possible, for example a spherical shape to supporta member making a tumbling movement.

In general, such a gas bearing system must have a relatively highstiffness, but there must also be an effective damping of vibrations inthe bearing system, especially in case the gas bearing system is used inhigh precision machines, like coordinate measuring machines.

However, the volume of the bearing gap in combination with thecompressibility of the gas may cause a delay in the response of thebearing pressure to a change in the distance between the bearingsurfaces. This delay introduces a negative phase shift, which may resultin an unstable bearing system, depending on the frequency of the changeof said distance, whereby so called pneumatic hammering may occur.

To increase the load capacity of gas bearing systems, the bearing gapmay have a chamber, i.e. a recessed area in one of the bearing surfaces.In said recessed area the distance between the two bearing surfaces islarger than the distance between said two bearing surfaces in theportion of the bearing gap surrounding said recessed area. In thatsurrounding portion said distance may be for example between 0.005 mmand 0.01 mm, while in the chamber said distance is for example between0.01 mm and 0.05 mm. Although such chamber increases -the load capacityof the bearing system, it may increase the instability of the gasbearing system caused by the compressibility of the gas in the bearinggap.

The object of the invention is to provide an improvement of gas bearingsystems resulting in effective damping of vibrations in the system.

In order to accomplish that objective, the bearing system comprises acavity having a content between 0.001 cm³ and 0.2 cm³, preferablybetween 0.001 cm³ and 0.1 cm³, which cavity is connected to the bearinggap through an orifice. Preferably the diameter of the orifice isbetween 0.05 mm and 0.3 mm, more preferably between 0.1 mm and 0.2 mm.

The cavity is closed at all sides and communicates with the outside onlythrough said orifice, which orifice restricts the gas flow to and fromthe cavity. If the content of the cavity is so small that a substantialchange in gas pressure is generated inside the cavity in response to thechanging (vibrating) gas pressure outside the cavity, then the presenceof the cavity is able to damp vibrations of the bearing system.Depending on dimensions of the bearing, and the content of the cavity,and the diameter of the orifice, a certain frequency range of vibrationscan be damped. For each application the optimal dimensions can easily befound by experimentation.

Although the invention can be advantageously applied in any kind of gasbearing system, very good results are obtained in gas bearing systemswhere one of the bearing surfaces comprises a recessed area in which thedistance between the two bearing surfaces is larger than the distancebetween said two bearing surfaces in the portion of the bearing gapsurrounding said area.

Preferably, more than one cavity is connected to the bearing gap toachieve a more effective damping action. The contents of the differentcavities may be equal, but in one preferred embodiment the cavities havea different content, the difference being more than 10%, preferably morethan 20%, more preferably more than 50%. By making use of cavities withmutual different contents, a larger frequency range of vibrations ordifferent frequency ranges of vibration can be damped. Furthermore theorifices may have different dimensions, adapted to the dimensions of thebearing gap and the frequency range.

Each cavity may be connected directly with said bearing gap through anorifice, but in another preferred embodiment one of the cavities isconnected to another cavity through an orifice, so that said one of thecavities is connected with the bearing gap through said other cavity.Also more than two cavities may be interconnected through orifices,enabling further tuning of the damping action.

The invention will now be explained in more detail by means of adescription of four embodiments of a gas bearing system provided withflat bearing surfaces, in which reference is made to a drawing, inwhich:

FIG. 1 is a sectional view of a gas bearing comprising one cavity;

FIG. 2 is a sectional view of a gas bearing, one bearing surface beingprovided with a chamber;

FIG. 3 is a sectional view of a gas bearing comprising two cavities; and

FIG. 4 is a sectional view of another gas bearing comprising twocavities.

The figures are schematic representations of the embodiments, in whichsome dimensions are out of proportion to achieve a better representationof relevant details. All four figures show a cross section perpendicularto the plane of the substantially flat bearing surfaces.

FIG. 1 shows a first bearing member 1 having a flat first bearingsurface 2, and a second bearing member 3 having a flat second bearingsurface 4 opposing said first bearing surface 2. The two bearingsurfaces 2,4 are parallel. The bearing members 1,3 may be made frommetal or plastic or another material.

Between the two bearing surfaces 2,4 there is a bearing gap 5 into whichair, or another gas, is brought through air supply duct 6 in bearingmember 1. Air supply duct 6 terminates near bearing surface 2 and isconnected with bearing gap 5 by an orifice 7 restricting the airflow.

Because of the air pressure in bearing gap 5 the second bearing member 3is supported by the first bearing member 1 without contact between thetwo bearing surfaces 2,4. The air cushion in the bearing gap 5 keeps thetwo bearing members 1,3 apart. The air will escape at the edge 8 of thebearing gap 5, but new compressed air will be supplied to the bearinggap 5 by air duct 6 in order to keep the required air pressure inbearing gap 5.

The first bearing member 1 can be present at a fixed location in amachine, while the second bearing surface 4 of the second bearing member3 can move over the fixed first bearing surface 2 to guide and supportanother part of the machine connected to second bearing member 3. Secondbearing member 3 is supported by the air cushion in the bearing gap 5between the two bearing surfaces 2,4.

More than one orifice 7 can be present to supply air to the bearing gap5 between the two bearing surfaces 2,4 to maintain the air cushion. Itis also possible to provide the moving bearing member 3 with an airsupply duct instead of the air supply duct 6 in bearing member 1, oradditional to air supply duct 6.

The dimensions of the bearing system can be as follows. The bearingsurfaces 2,4 may have a dimension of about 20 cm². The distance betweenthe two bearing surfaces 2,4 can be between 0.005 mm and 0.01 mm. Thediameter of the orifice 7 can be between 0.1 mm and 0.2 mm, and itslength is for example 1 mm.

FIG. 1 shows an embodiment of the gas bearing system comprising onecavity 10 in the first bearing member 1. Cavity 10 is closed at allsides and is connected with the bearing gap by orifice 11. Orifice 11restricts the airflow from bearing gap 5 to cavity 10 and from cavity 10to bearing gap 5.

The content of cavity 10 is for example 0.05 cm³ and the orifice 11 hasfor example a diameter of 0.1 mm and a length of 1 mm. The cavity 10 canbe manufactured by drilling a blind bore in the bearing surface 2 ofbearing member 1 and filling the entrance of the bore with a covercomprising the orifice 11. Depending on the design other ways formanufacturing the cavity are obvious.

The dimension of the cavity 10 and the orifice 11, in combination withthe dimensions and characteristics of the bearing, will result in adamping effect on vibrations of the bearing members 1,3 relative to eachother, within a certain frequency range. The optimal dimensions have tobe found by experiments rather then by calculations.

FIG. 2 shows an embodiment of a gas bearing system wherein first bearingmember 1 is provided with a chamber 13, i.e. a recessed area in bearingsurface 2. In the recessed area (chamber 13) the distance between thetwo bearing surfaces 2,4 is larger than the distance between said twobearing surfaces 2,4 in the portion of the bearing gap 5 surrounding therecessed area 13. In the surrounding portion said distance is forexample between 0.005 mm and 0.01 mm, while in the chamber 13 saiddistance is for example between 0.01 mm and 0.05 mm.

Because of the presence of the chamber 13 the average air pressure inthe bearing gap 5 will be higher, so that the same air supply pressurewill result in a higher load capacity of the bearing system.

As shown in FIG. 2, the cavity 10 is connected by orifice 11 with therecessed area (chamber 13) of the first bearing surface 2. Thedimensions of cavity 10 and orifice 11 can be the same as mentionedabove for the embodiment shown in FIG. 1.

FIG. 3 shows an embodiment of a gas bearing system comprising twocavities 14,15, each being connected by an orifice 16,17 to the chamber13 of the bearing gap 5. The content of cavity 14 is twice the contentof cavity 15, so that different frequency ranges shall be damped.

FIG. 4 shows another embodiment of a gas bearing system wherein twocavities 18,19 are present. Cavity 18 is connected by orifice 20 tocavity 19, and cavity 19 is also connected to bearing gap 5 by orifice21. Such configuration provides possibilities for further tuning thedamping action.

The embodiments as described above are merely examples; a great manyother embodiments are possible, for example gas bearing systems havingcylindrical bearing surfaces, where one of the bearing members rotatesaround the axis of the cylinder and cavities are present in at least oneof the bearing surfaces for damping vibrations in the system. Also othershapes—adapted to the relative movement of the bearing surfaces 2,4—arepossible, for example a spherical shape to support a bearing member 3making a tumbling movement.

1. A gas bearing system comprising two opposing and substantiallyparallel bearing surfaces (2,4) and at least one gas duct (6) forsupplying gas to the bearing gap (5) between said bearing surfaces(2,4), characterized by a cavity (10,14,15,18,19) having a contentbetween 0.001 cm³ and 0.2 cm³, which cavity (10,14,15,18,19) isconnected to said bearing gap (5) through an orifice (7,11,16,17,20,21).2. A gas bearing system as claimed in claim 1, characterized in that thecontent of the cavity (10,14,15,18,19) is between 0.001 cm³ and 0.1 cm³.3. A gas bearing system as claimed in claim 1, characterized in that thediameter of said orifice (7,11,16,17,20,21) is between 0.05 mm and 0.3mm, preferably between 0.1 mm and 0.2 mm.
 4. A gas bearing system asclaimed in claim 1, characterized in that one of said bearing surfaces(2) comprises a recessed area (13) in which the distance between saidtwo bearing surfaces (2,4) is larger than the distance between said twobearing surfaces (2,4) in the portion of said bearing gap (5)surrounding said recessed area (13).
 5. A gas bearing system as claimedin claim 1, characterized in that more than one cavity (10,14,15,18,19)is connected to said bearing gap (5).
 6. A gas bearing system as claimedin claim 5, characterized in that the cavities (10,14,15,18,19) have adifferent content, the difference being more than 10%, preferably morethan 20%, more preferably more than 50%.
 7. A gas bearing system asclaimed in claim 5, characterized in that each cavity (10,14,15,19) isconnected directly with said bearing gap through an orifice(7,11,16,17,21).
 8. A gas bearing system as claimed in claim 5,characterized in that one of the cavities (18) is connected to anothercavity (19) through an orifice (20).
 9. A gas bearing system as claimedin claim 8, characterized in that more than two cavities (18,19) areinterconnected through orifices (20).